Trap catch as a relative index of ferret (Mustela furo) abundance in a New Zealand pastoral habitat

The density of a ferret population in South Island pastoral farmland was estimated by live capture, mark and release, and by removal trapping, during March and April 1997. An estimate of 6.3 ferrets per km(2) (5.5-8.4, 95% CI) was obtained after 8 days of mark and release, whereas 10 days of removal trapping provided an estimate of 5.3 ferrets per km(2) (5.3-5.9, 95% CI). There was significant variation in individual trappability. Proportionally more unmarked ferrets were captured as population density was reduced during removal trapping. Four person-days effort were required to remove 50% of the ferret population from an area of 1254 ha, while removal of 80 or 95% of the population took considerably greater effort. Trapping provided a reliable relative index of ferret abundance. While there is a need to replicate this study in order to confirm its validity, it confirms the use of a relative index as a valuable tool for measuring the efficacy of control operations and estimating ferret abundance.